Light emitting device with improved extraction efficiency

ABSTRACT

In embodiments of the invention, a semiconductor structure comprising a III-nitride light emitting layer disposed between an n-type region and a p-type region is grown on a substrate. The substrate is a non-III-nitride material. The substrate has an in-plane lattice constant a substrate . At least one III-nitride layer in the semiconductor structure has a bulk lattice constant a layer  and [(|a substrate −a layer |)/a substrate ]*100% is no more than 1%. A surface of the substrate opposite the surface on which the semiconductor structure is grown is textured.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/882,511, filed Oct. 21, 2013, to be issued as U.S. Pat. No. 9,209,359on Dec. 8, 2015, which is a 371(c) national stage entry ofPCT/IB2011/054847 filed Nov. 1, 2011, which is the internationalapplication of U.S. Provisional Patent Application Ser. No. 61/409,160,filed Nov. 2, 2010. U.S. Pat. No. 9,209,359 and PCT/IB2011/054847 areincorporated herein by reference.

BACKGROUND

Field of Invention

The present invention relates to a III-nitride light emitting deviceincluding a light extraction feature.

Description of Related Art

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

Since native III-nitride substrates are generally expensive and notwidely available, III-nitride devices are often grown on sapphire or SiCsubstrates. These non-III-nitride substrates are less than optimalbecause sapphire and SiC have different lattice constants than theIII-nitride layers grown on them, causing strain and crystal defects inthe III-nitride device layers, which can cause poor performance andreliability problems. In addition, light may be trapped inside theIII-nitride structure by a waveguide formed at the interface between theIII-nitride material and these non-III-nitride substrates.

SUMMARY

It is an object of the invention to provide a III-nitride device with alight extraction feature.

In embodiments of the invention, a semiconductor structure comprising aIII-nitride light emitting layer disposed between an n-type region and ap-type region is grown on a substrate. The substrate is anon-III-nitride material and has an in-plane lattice constanta_(substrate). At least one III-nitride layer in the semiconductorstructure has a bulk lattice constant a_(layer) and[(|a_(substrate)−a_(layer)|)/a_(substrate)]*100% is no more than 1%. Asurface of the substrate opposite the surface on which the semiconductorstructure is grown is textured. The texturing may improve lightextraction.

In a method according to embodiments of the invention, a semiconductorstructure comprising a III-nitride light emitting layer disposed betweenan n-type region and a p-type region is grown on a substrate. Thesubstrate is removed. The substrate is a non-III-nitride material withan in-plane lattice constant a_(substrate). At least one III-nitridelayer in the semiconductor structure has a bulk lattice constanta_(layer) and [(|a_(substrate)−a_(layer)|)/a_(substrate)]*100% is nomore than 1%. A thickness of the semiconductor structure is selected toreduce a number of guided optical modes within the semiconductorstructure, which may improve light extraction.

In a method according to embodiments of the invention, a substrate witha patterned surface is provided. The patterned surface comprises atleast one region of lower elevation where a portion of the substrate isremoved. A semiconductor structure comprising a III-nitride lightemitting layer disposed between an n-type region and a p-type region isgrown on the patterned surface, such that semiconductor material fillsin the region of lower elevation. The substrate is removed such that thesemiconductor material filling in the region of lower elevation remainspart of the semiconductor structure, which may improve light extractionfrom the device.

The devices described herein may have less strain and therefore betterperformance than conventionally grown III-nitride light emittingdevices. A light extraction feature, such as texturing or selecting athickness to reduce guided optical modes in the device, is provided toimprove light extraction from the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of a flip chip III-nitride device includinga textured substrate.

FIG. 2 illustrates a portion of a vertical current injection III-nitridedevice including a textured substrate.

FIG. 3 illustrates a portion of a III-nitride device grown over lightextraction features formed on a substrate.

FIG. 4 illustrates a portion of a III-nitride device with a thicknessselected to maximize extraction efficiency.

FIG. 5 illustrates a III-nitride device formed in the shape of atruncated inverted pyramid.

FIGS. 6A and 6B illustrate portions of III-nitride semiconductorstructures including embedded light extraction features.

DETAILED DESCRIPTION

The sapphire and SiC substrates upon which III-nitride LEDs areconventionally grown cannot be easily etched by wet chemistries. Thus,in order to form textured light extraction surfaces, the substrate isgenerally removed by a method such as laser lift-off in the case ofsapphire. The III-nitride material exposed by removing the substrate maythen be textured. Removing the substrate requires additional processingsteps and can reduce yield. For example, laser lift-off can damage theIII-nitride material, which may reduce manufacturing yields.

In embodiments of the invention, a substrate is provided which islattice matched (or nearly so) to at least one III-nitride layer in thedevice. In some embodiments, the substrate has the same hexagonalsymmetry as III-nitride material. Since the substrate is substantiallylattice matched, the III-nitride structure grown on the substrate may beless strained than in a conventional device. In addition, in someembodiments, the substrate can be etched with a wet chemical etch toform light extraction features. In some embodiments, a wet chemical etchmay be used to selectively etch the substrate from the III-nitridestructure.

In various embodiments, light extraction may be improved by lightextraction features etched into the substrate prior to deposition of theIII-nitride structure, by light extraction features etched into thesubstrate after deposition of the III-nitride structure, and by growinga thin (<1 μm total thickness) III-nitride structure, then removing thesubstrate.

A III-nitride layer in a device may be characterized by a bulk latticeconstant and an in-plane lattice constant. The bulk lattice constant isthe lattice constant of a relaxed material with the same composition asthe III-nitride layer. The in-plane lattice constant is the latticeconstant of the III-nitride layer as grown in the device. If theIII-nitride layer as grown in the device is strained, the bulk latticeconstant is different from the in-plane lattice constant. The substrateon which the III-nitride structure is grown is a non-III-nitridematerial with an in-plane lattice constant within 1% of the bulk latticeconstant of at least one deposited III-nitride layer in someembodiments, and within 0.5% of the bulk lattice constant of at leastone deposited III-nitride in some embodiments. In other words,[(|a_(substrate)−a_(layer)|)/a_(substrate)]*100% is no more than 1% insome embodiments, no more than 0.5% in some embodiments, and no morethan 0.1% in some embodiments. In some embodiments, the substrate hassimilar or the same hexagonal wurtzite symmetry as the III-nitridestructure. In some embodiments, the substrate is substantiallyimpervious to attack by the chemical and thermal environment experiencedduring the deposition of the III-nitride structure. In some embodiments,the substrate has an in-plane coefficient of thermal expansion within30% of that of the III-nitride structure. In some embodiments, thesubstrate is a single crystal or substantially single crystal material.

In some embodiments, the substrate is a material of general compositionRAO₃(MO)_(n), where R is a trivalent cation, often selected from Sc, In,Y, and the lanthanides (atomic number 57-71); A is also a trivalentcation, often selected from Fe (III), Ga, and Al; M is a divalentcation, often selected from Mg, Mn, Fe (II), Co, Cu, Zn and Cd; and n isan integer ≧1. In some embodiments, n≦9 and in some embodiments, n≦3. Insome embodiments, RAMO₄ (i.e., n=1) compounds are of the YbFe₂O₄structure type, and RAO₃(MO)_(n) (n≧2) compounds are of theInFeO₃(ZnO)_(n) structure type.

Examples of suitable substrate materials include the followingmaterials:

Lattice y in lattice-matched Material constant a (Å) AppearanceAl_(x)In_(y)Ga_(1−x−y)N, x = 0 InFeZn₂O₅ 3.309 Brown 0.34 InFeZn₈O₁₁3.276 Brown 0.25 ScGaMgO₄ 3.272 Transparent 0.24 ScAlMgO₄ 3.236Transparent 0.14 InAlMgO₄ 3.29 Transparent 0.29 ScAlMnO₄ 3.26Transparent 0.20 InFeMnO₄ 3.356 Brown 0.48 InAlMnO₄ 3.319 Black 0.37InAlCoO₄ 3.301 Black 0.32 InGaFeO₄ 3.313 Black 0.36

These and related substrate materials are described in detail byKimizuka and Mohri in “Structural Classification of RAO₃(MO)_(n)Compounds (R═Sc, In, Y, or Lanthanides; A=Fe(III), Ga, Cr, or Al;M=Divalent Cation; n=1-11)”, published in Journal of Solid StateChemistry 78, 98 (1989), which is incorporated herein by reference.

A III-nitride structure is deposited on the substrate by any of themeans known in the art, including, for example, MOCVD, hydride vaporphase epitaxy, or MBE. Perfect lattice match between the III-nitridestructure and the substrate is not necessary, although lattice matchwithin 0.1% is desirable in some embodiments. For purposes ofembodiments of the present invention, the bulk lattice constant of aternary or quaternary AlInGaN layer may be estimated according Vegard'slaw, which for Al_(z)In_(y)Ga_(z)N may be expressed asa_(AlInGaN)=x(a_(AlN))+y(a_(InN))+z(a_(GaN)), where the variable “a”refers to the bulk a-lattice constant of each binary material andx+y+z=1. AlN has a bulk lattice constant of 3.111 Å, InN has a bulklattice constant of 3.544 Å, and GaN has a bulk lattice constant of3.1885 Å.

In some embodiments, the III-nitride structure is grown on a surface ofthe substrate that is “miscut” or angled relative to a majorcrystallographic plane of the substrate. In some embodiments, thesurface of the substrate on which the III-nitride structure is grown maybe oriented between −10 and +10 degrees away from the basal (0001)plane. In some embodiments, miscuts between −0.15 and +0.15 degreestilted from the (0001) plane may result in large atomic terraces on thesubstrate surface that may desirably reduce the number of defects formedat terrace edges.

Though in the examples below the semiconductor device structure is aIII-nitride LED that emits blue or UV light, electronic andoptoelectronic devices besides LEDs such as laser diodes, high electronmobility transistors, and heterojunction bipolar transistors may beformed according to embodiments described herein.

As a preliminary matter, a semiconductor structure 22 as illustrated inFIGS. 1-4 is grown over substrate 10. The semiconductor structure 22includes a light emitting or active region 26 sandwiched between n- andp-type regions 24 and 28. An n-type region 24 is often grown first andmay include multiple layers of different compositions and dopantconcentration including, for example, preparation layers such as bufferlayers or nucleation layers, which may be n-type or not intentionallydoped, and n- or even p-type device layers designed for particularoptical or electrical properties desirable for the light emitting regionto efficiently emit light. A light emitting or active region 26 is grownover the n-type region 24. Examples of suitable light emitting regions26 include a single thick or thin light emitting layer, or a multiplequantum well light emitting region including multiple thin or thicklight emitting layers separated by barrier layers. A p-type region 28 isgrown over the light emitting region 26. Like the n-type region 24, thep-type region 28 may include multiple layers of different composition,thickness, and dopant concentration, including layers that are notintentionally doped, or n-type layers.

In the embodiments described below, etching or texturing steps may beperformed by any suitable etch such as for example, wet chemicaletching. For example, ScMgAlO₄ is readily attacked by aqueous mixturesof H₃PO₄ and H₂O₂, H₂SO₄:H₂O₂:H₂O, and aqueous mixtures of HF, asreported by C. D. Brandle, et al. in “Dry and Wet Etching of ScMgAlO₄”published in Solid-State Electronics, 42, 467 (1998), which isincorporated herein by reference. In some embodiments, etching ortexturing is performed by reactive ion etching using a gaseous mixtureof Cl₂ and Ar at an applied power of 800 Watts.

In the device illustrated in FIG. 1, the III-nitride structure 22 isgrown on face 11 of substrate 10. The total thickness of thesemiconductor structure 22 is between 2 and 100 μm in some embodimentsand between 3 and 6 μm in some embodiments. The face 12 of substrate 10opposite the III-nitride structure 22 is textured, for example byroughening, which is random, or patterning, which may be ordered orrandom, to enhance the extraction of light from the III-nitridestructure 22. In some embodiments, face 12 is textured by forminghexagonal pyramids. The pyramids have bases between 100 nm and 1 μm widein some embodiments and may be spaced between 100 nm and 1 μm apart insome embodiments. The pyramids may be formed naturally during wetetching. In some embodiments, face 12 is textured by conventionalpatterning. For example, face 12 may be etched to form a lattice ofposts, or a lattice of holes. The posts or holes may be arranged in anysuitable lattice such as a triangular, square, hexagonal, or Archimedeanlattice. The posts or holes may be between 100 nm and 1 μm wide in someembodiments, between 100 nm and 1 μm deep or tall in some embodiments,and spaced between 100 nm and 1 μm apart in some embodiments. In someembodiments, a mask with a ‘random’ pattern is formed over thesubstrate, then the substrate is etched with, for example, a wet etch. Amask with a random pattern may be formed by, for example, oxidizing ametal such as aluminum. Face 12 may be textured before or afterIII-nitride structure 22 is grown. Face 12 may be roughened by, forexample, mechanical means such as grinding or by wet etching. Face 12may be patterned by, for example, conventional lithographic patterningand wet etching. When the refractive index of substrate 10 is close tothe refractive index of III-nitride structure 22, the extraction benefitprovided by roughening or patterning face 12 is greater than when therefractive index of substrate 10 is much less than the refractive indexof III-nitride structure 22. GaN has a refractive index of 2.4.Accordingly, the refractive index of the substrate 10 is at least 2.0 insome embodiments, at least 2.2 in some embodiments, and at least 2.4 insome embodiments.

After growth of III-nitride structure 22, a metal p-contact 30 is formedon the p-type region 28, then a mesa structure 13 is etched into theIII-nitride structure 22, exposing a portion of the n-type region 24 onwhich a metal n-contact 32 is formed. One or both of n- and p-contacts30 and 32 may be reflective. The device may be mounted in a flip-chipconfiguration on any suitable mount (not shown in FIG. 1) such thatlight is extracted from the device through face 12 of substrate 10.

In the device illustrated in FIG. 2, a III-nitride structure 22 is grownon the face 11 of substrate 10. A metal p-contact 34 is deposited on thep-type region 28. The face 12 of substrate 10 opposite the III-nitridestructure 22 is patterned with one or more vias 14 formed by masking thesubstrate using standard photolithographic techniques and using, forexample, a solution containing HF or any other suitable etch toselectively etch holes through the substrate all the way to the n-typeregion 24. Though only one via 14 is illustrated in FIG. 2, many viasmay be formed on a single device. Vias 14 may be, for example, at least100 nm wide in some embodiments and between 100 nm and 5 μm wide in someembodiments. Neighboring vias 14 may be spaced at least 50 μm apart insome embodiments, between 50 μm and 500 μm apart in some embodiments,and between 100 μm and 300 μm apart in some embodiments. Currentspreading in the device favors wider vias more narrowly spaced, whileavoiding loss of light by shadowing caused by the vias favors narrowervias more widely spaced. In some embodiments, substrate 10 may bethinned before forming vias 14. For example, substrate 10 may be on theorder of 1 mm thick before thinning in some embodiments, and may bethinned to a thickness that is mechanically self supporting, for examplebetween 100 μm and 500 μm in some embodiments.

A metal n-contact 36 is formed on the surface 15 of the n-type region 24exposed by forming via 14. N-contact 36 may cover all or a portion ofthe sidewall of via 14 and a portion of the surface 12 of substrate 10,as illustrated in FIG. 2. Substrate surface 12 may be textured asillustrated in FIG. 2, before or after forming via 14. The portion ofsubstrate surface 12 under n-contact 36 may be textured as illustratedin FIG. 2, or it may be left untextured, for example by conventionalmasking steps. The device may be attached to a mount through p-contact34. Electrical connection to n-contact 36 may be made by any suitableconnection such as a wire-bond. P-contact 34 may be reflective. Light isextracted from the device through surface 12 of substrate 10. In someembodiments, the device of FIG. 2 is connected to a mount throughp-contact 34 on a wafer scale, then substrate 10 is thinned to athickness that is not mechanically self-supporting, for example 50 μm orless in some embodiments, 20 μm or less in some embodiments, and 10 μmor less in some embodiments. Via 14 may be formed before or afterthinning. Though the remaining thin portion of substrate 10 may not bemechanically self supporting, it may provide mechanical robustness whichmay protect the semiconductor structure 22 during subsequent processingsteps. For example, the remaining thin portion of substrate 10 mayprotect the semiconductor structure from damage when a bond pad and bondare formed on n-contact 36.

In the device illustrated in FIG. 3, the face 11 of substrate 10 onwhich semiconductor structure 22 is grown is patterned prior todeposition of the semiconductor structure 22. The pattern may include atleast one region of lower elevation where the substrate is removed, forexample a hole or a region between two posts. The pattern may beproduced by creating a mask using standard photolithographic techniquesand then etching holes or posts 38 into the substrate with, for example,a solution containing HF or any other suitable etch. Holes or posts 38may be between 100 nm and 1 μm wide in some embodiments and between 100nm and 1 μm deep or tall in some embodiments. Semiconductor structure 22is then grown on substrate 10 as described above. In some embodimentssemiconductor material fills the holes or fills in the space betweenposts. The difference in refractive index between substrate 10 and thesemiconductor material filling holes 38 or filling the space betweenposts 38 may cause scattering which may improve light extraction fromthe device. In some embodiments, holes or posts 38 are arranged in alattice and form a photonic crystal. In a conventional device, due tothe lattice constant difference between the semiconductor material andthe substrate, the n-type region must be grown thick in order to begrown with sufficiently high quality. In the device of FIG. 3, becausethe n-type region 24 is more closely lattice-matched to the substrate10, the n-type region 24 may be thinner than in a conventional device,which advantageously places the photonic crystal closer to the lightemitting region 26. For example, in a conventional device, the n-typeregion may be at least 5 μm thick. In the device of FIG. 3, the n-typeregion may be between 500 nm and 2 μm thick.

In some embodiments, a light extraction structure is embedded in asemiconductor layer of the device, as illustrated in FIGS. 6A and 6B.Though FIGS. 6A and 6B illustrate a light extraction structure embeddedin n-type region 24, a light extraction structure may be embedded in anysemiconductor layer in the device. In the portion of the semiconductorstructure illustrated in FIG. 6A, a first portion 24 a of n-type region24 is grown, then a pattern 52 is formed, for example by etching portion24 a or selectively growing portion 24 a, for example through apatterned mask. Pattern 52 may include at least one region of lowerelevation, for example a hole or a region between two posts. Pattern 52may be holes or posts, which in some embodiments are arranged in alattice and form a photonic crystal. After pattern 52 is formed, asecond portion 24 b of n-type region 24 is grown over pattern. In someembodiments materials 24 a and 24 b have different indices of refractionand semiconductor material 24 b fills the holes or fills in the spacebetween posts. The difference in refractive index between first portion24 a and second portion 24 b may cause scattering which may improvelight extraction from the device. For example, in a device grown on aScMgAlO₄ substrate, first portion 24 a may be one of In_(0.14)Ga_(0.86)Nand In_(0.3)Al_(0.7)N and second portion 24 b may be the other ofIn_(0.14)Ga_(0.86)N and In_(0.3)Al_(0.7)N. The features of pattern 52may have a width and depth of between 50 and 1000 nm in someembodiments. In some embodiments, second material 24 b is grown suchthat voids are formed in pattern 52, which may cause scattering whichmay improve light extraction from the device. In the portion of thesemiconductor structure illustrated in FIG. 6B, a first portion of then-type region is grown, regions of a material with a different index 54are formed over the first portion of the n-type region, then a secondportion of the n-type region is grown over the regions of material withdifferent index 54. Regions of material with a different index 54 maybe, for example, an oxide of silicon, a nitride of silicon, SiO₂, Si₃N₄,or air.

The device illustrated in FIG. 3 may be processed into either the flipchip configuration illustrated in FIG. 1 or the vertical currentinjection configuration illustrated in FIG. 2. In some embodiments, thesurface 12 of substrate 10 from which light is extracted from the devicemay also be textured, as described above in reference to FIGS. 1 and 2.Texturing both faces 11 and 12 of substrate 10 may result in greaterlight extraction efficiency than texturing only one surface asillustrated in FIGS. 1 and 2.

In some embodiments, substrate 10 of FIG. 3 is removed fromsemiconductor structure 22, for example by etching with an etch thatremoves substrate 10 without attacking the III-nitride semiconductorstructure. The surface of semiconductor structure 22 exposed by etchingaway substrate 10 includes a pattern of semiconductor material formedduring growth of semiconductor structure 22, when the semiconductormaterial filled the holes or filled in the space between the posts onthe patterned surface 11 of substrate 10.

In one example of a suitable semiconductor structure 22 for the devicesillustrated in FIGS. 1, 2, and 3, a layer 24 of n-typeIn_(0.12)Ga_(0.88)N is deposited on a ScMgAlO₄ substrate 10. Alight-emitting layer 26 formed of In_(0.14)Ga_(0.86)N is deposited onthe n-type layer 24. A p-type layer stack 28 formed of GaN andIn_(0.12)Ga_(0.88)N is deposited on the light-emitting layer 26. TheScMgAlO₄ substrate 10 may be randomly textured by etching in a solutionof H₂SO₄:H₂O for 10 minutes, or patterned by conventional lithographicsteps, then etched with HF.

In structures where the substrate 10 remains a part of the finisheddevice, such as the devices illustrated in FIGS. 1, 2, and 3, in someembodiments the substrate 10 alone or both the substrate 10 and thesemiconductor structure 22 are shaped, for example to improve lightextraction. FIG. 5 illustrates a device formed into the shape of atruncated inverted pyramid. As illustrated in FIG. 5, the top and sidesurfaces of the substrate 10 form an acute angle. In some embodiments,the top and side surfaces of the substrate 10 may form an obtuse angle,or may be joined by curved surface. A device may be shaped by, forexample, beveled saw cuts, shaping with a sharp blade, grinding,etching, or some of the methods for removing the substrate describedbelow. In some embodiments, substrate 10 of FIG. 5 is transparent andthe device is a flip chip such that light is extracted from the devicethrough substrate 10.

In the device illustrated in FIG. 4, the light extraction feature is thethickness of the semiconductor structure 22, which is selected tomaximize the extraction efficiency by minimizing the number of guidedoptical modes within the structure. The thickness depends on therefractive index of the semiconductor material and the substrate, andthe wavelength of emitted light in the semiconductor. The totalthickness of semiconductor structure 22, including all III-nitridelayers in the device, is between 200 nm and 2 μm in some embodiments andbetween 500 nm to 1 μm in some embodiments. In the device illustrated inFIG. 4, a semiconductor structure 22 is grown on a substrate asdescribed above. A metal p-contact 40 is deposited on the p-type region28. The structure including the substrate, semiconductor structure 22,and p-contact 40 is attached to a mount (not shown in FIG. 4) throughp-contact 40, for example by a metal-metal bond or by solder.

The substrate is then removed by any suitable method or combination ofmethods. A transparent substrate may be removed by laser lift off, wherethe layer of III-nitride material grown first on the substrate absorbsthe laser light and melts, releasing semiconductor structure 22 from thesubstrate. Laser lift-off may be facilitated by an optional layer ofnarrower-energy-gap material grown first on the substrate. Thecomposition of the narrower-energy-gap layer may be selected such thatit absorbs more of the incident laser light than the rest ofsemiconductor structure 22, which may reduce the incident flux requiredand may produce less distributed damage throughout the semiconductorstructure 22.

In some embodiments, an optional zone of weakness is provided at or nearthe interface of the III-nitride material and the substrate in order toencourage the fracture of that interface and thereby make it easier toremove the semiconductor structure 22 from the substrate. A zone ofweakness may be provided in the substrate or semiconductor structure 22by implanting one or more of H or N or other atoms prior to or afterdeposition of all or part of the semiconductor structure 22. A zone ofweakness in semiconductor structure 22 may be provided by first growinga layer with a higher mole fraction of InN (at a certain growthtemperature), then growing a layer with a lower mole fraction of InN.The higher-InN-bearing layer may transform at higher growth temperaturesused to grow the remaining semiconductor structure 22 according to itsphase diagram into regions of even higher and lower indium composition.The regions of highest indium composition are more absorbent of incidentlaser light, and the mechanical stress due to the spatially varyingindium composition may create a layer of mechanical weakness in thesemiconductor structure.

A zone of weakness may also be provided at the semiconductorstructure/substrate interface by exposing the wafer to a pattern oftightly focused, pulsed laser beams of sufficient intensity and photonenergy to create a plurality of micron-scale crystal defects or voids inthe crystalline structure. The pattern of crystal damage may begenerated by rastering one or more laser beams across the wafer, or byusing diffractive optics to generate a large number of spots from asingle high power laser such as an excimer laser. The laser beams may bestrongly converging with a short sub-microsecond pulse, and may createhighly localized damage. This exposure could occur through the epitaxystack after growth with sufficiently low dose that further waferprocessing may be done after exposure. In some embodiments, thesubstrate is removed after the subsequent wafer processing, for examplein a die-level rather than a wafer-level process. Also, the total powerof the exposure may be less than required for traditional laserlift-off, which may result in less mechanical shock.

A zone of weakness may also be provided at the semiconductorstructure/substrate interface by patterning the surface of the substrate(e.g. with a rectangular or triangular lattice of ridges of substratematerial) prior to deposition of the semiconductor structure 22.

Substrate materials shown in the table above have micaceous character,wherein the basal plane of the crystal (i.e. the face parallel to thesurface of the substrate in the case where the substrate orientation is(0001)) preferentially fractures. Such substrates may be removed bymechanical methods including but not limited to mechanical grinding,applying a rotational force between the substrate and the semiconductorstructure, attaching an adhesive-coated plastic film to the substrateand a second adhesive-coated plastic film to the semiconductor structureand pulling the substrate and semiconductor structure apart, using asharp blade to break the interface between the substrate and thesemiconductor structure, using a pulse of sonic energy or inhomogeneoustemperature distribution to break the interface between the substrateand the semiconductor structure, and applying a temperature gradientacross the surface normal of the semiconductor structure and substrate(for example, higher temperature applied to one face of thesemiconductor structure, and lower temperature applied to one face ofthe substrate), such that the thermally induced stress in the plane ofthe semiconductor structure/substrate interface is sufficient to causefracture of that interface.

In some embodiments, the substrate is removed by wet chemical etching.For example, ScMgAlO₄ is readily attacked by aqueous mixtures of H₃PO₄and H₂O₂, H₂SO₄:H₂O₂:H₂O, and aqueous mixtures of HF, as reported by C.D. Brandle, et al. in “Dry and Wet Etching of ScMgAlO4” published inSolid-State Electronics, 42, 467 (1998). In some embodiments, all orpart of growth substrate 30 is removed by reactive ion etching using agaseous mixture of Cl₂ and Ar at an applied power of 800 Watts. In someembodiments, the final removal step is a wet chemical etch with HF. Insome embodiments, after removing the substrate, the surface of thesemiconductor material may be treated, for example to planarize thesurface and/or to remove damage caused by etching. For example, thesurface may be planarized by grinding, then cleaned with a wet etch.

A layer of conductive, transparent metal oxide 42 such as indium tinoxide, zinc oxide, magnesium-doped zinc oxide, aluminum-dopedmagnesium-doped zinc oxide, gallium-doped magnesium-doped zinc oxide,aluminum-doped zinc oxide, gallium-doped zinc oxide, or ruthenium oxide,for example, is deposited on the surface of n-type region 24 exposed byremoving the substrate. Conductive layer 42 may form an n-contact withsufficient current spreading capability. In some embodiments, conductivelayer 42 serves as a current spreading layer and a separate metaln-contact 44 is formed on conductive layer 42. Conductive layer 42 maybe between 100 nm and 1 μm thick in some embodiments and between 100 nmand 300 nm thick in some embodiments. Light is extracted from the devicethrough conductive layer 42.

In one example of a device as illustrated in FIG. 4, a layer of n-typeIn_(0.12)Ga_(0.88)N 24 is deposited on a ScMgAlO₄ substrate. Alight-emitting layer 26 formed of In_(0.14)Ga_(0.86)N is deposited onthe n-type layer 24. A p-type layer stack 28 formed of GaN andIn_(0.12)Ga_(0.88)N is deposited on the light-emitting layer 26.

The devices described above may be combined with one or more wavelengthconverting materials such as phosphors, quantum dots, or dyes to createwhite light or monochromatic light of other colors. All or only aportion of the light emitted by the LED may be converted by thewavelength converting materials. Unconverted light emitted by the LEDmay be part of the final spectrum of light, though it need not be.Examples of common combinations include a blue-emitting LED combinedwith a yellow-emitting phosphor, a blue-emitting LED combined withgreen- and red-emitting phosphors, a UV-emitting LED combined with blue-and yellow-emitting phosphors, and a UV-emitting LED combined withblue-, green-, and red-emitting phosphors. Wavelength convertingmaterials emitting other colors of light may be added to tailor thespectrum of light emitted from the device.

The wavelength converting element may be, for example, a pre-formedceramic phosphor layers that is glued or bonded to the LED or spacedapart from the LED, or a powder phosphor or quantum dots disposed in anorganic or inorganic encapsulant that is stenciled, screen printed,sprayed, sedimented, evaporated, sputtered, or otherwise dispensed ordeposited over the LED.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is being claimed is:
 1. A device comprising: a substrate; and asemiconductor structure grown on the substrate, the semiconductorstructure comprising a III-nitride light emitting layer disposed betweenan n-type region and a p-type region; wherein: the substrate is anon-III-nitride material; the substrate has an in-plane lattice constanta_(substrate); at least one III-nitride layer in the semiconductorstructure has a bulk lattice constant a_(layer);[(|a_(substrate)−a_(layer)|)/a_(substrate)]*100% is no more than 1%; anda surface of the substrate on which the semiconductor structure is grownis angled relative to a major crystallographic plane of the substrate.2. The device of claim 1 wherein the surface of the substrate on whichthe semiconductor substrate is grown is oriented between −10 and +10degrees away from a basal (0001) plane.
 3. The device of claim 1 whereinthe surface of the substrate on which the semiconductor substrate isgrown is angled such that large atomic terraces form on surface of thesubstrate on which the semiconductor substrate is grown.
 4. The deviceof claim 1 wherein a surface of the substrate opposite a surface onwhich the semiconductor structure is grown is textured.
 5. The device ofclaim 1 wherein the substrate is ScMgAlO₄.
 6. The device of claim 1wherein the substrate is RAO₃(MO)_(n), where R is selected from Sc, In,Y, and the lanthanides; A is selected from Fe (III), Ga, and Al; M isselected from Mg, Mn, Fe (II), Co, Cu, Zn and Cd; and n is an integer≧1.
 7. The device of claim 1 wherein the substrate has an index ofrefraction of at least 2.0.
 8. The device of claim 1 wherein the surfaceof the substrate on which the semiconductor structure is grown istextured.
 9. The device of claim 1 further comprising a lattice of holesor posts formed on the surface of the substrate on which thesemiconductor structure is grown.
 10. The device of claim 1 wherein thesubstrate is shaped into a truncated inverted pyramid.
 11. The device ofclaim 1 further comprising a photonic crystal formed in one of a surfaceof the substrate on which the semiconductor structure is grown and asemiconductor layer in the semiconductor structure.
 12. The device ofclaim 1 wherein the surface of the substrate opposite the surface onwhich the semiconductor structure is grown is textured with hexagonalpyramids.
 13. The device of claim 1 wherein the semiconductor structureis attached to a mount and the substrate is less than 50 μm thick.
 14. Adevice comprising: a substrate; a semiconductor structure grown on thesubstrate, the semiconductor structure comprising a III-nitride lightemitting layer disposed between an n-type region and a p-type region;and a pattern disposed within the n-type region, the pattern comprising:a first n-type layer with a pattern formed on a top surface of the firstn-type layer; and a second n-type layer disposed over the top surface ofthe first n-type layer, wherein the first and second n-type layers havedifferent indices of refraction; wherein: the substrate is anon-III-nitride material; the substrate has an in-plane lattice constanta_(substrate); at least one III-nitride layer in the semiconductorstructure has a bulk lattice constant a_(layer); and[(|a_(substrate)−a_(layer)|)/a_(substrate)]* 100% is no more than 1%.15. The device of claim 14 wherein the second n-type layer fills inregions between features that form the pattern.
 16. The device of claim14 wherein the pattern comprises at least one region of lower elevation.17. The device of claim 16 wherein the at least one region of lowerelevation is one of a hole and a region between two posts.
 18. Thedevice of claim 14 wherein the pattern comprises holes arranged in alattice to form a photonic crystal.
 19. The device of claim 14 whereinthe pattern comprises posts arranged in a lattice to form a photoniccrystal.